(1) Field of the Invention
The present invention relates to a CAD (Computer Aided Design) apparatus for aiding design of a printed wiring board, and in particular to placing components to effectively reduce noise, and confirmation of the placement of the components.
(2) Description of the Related Art
In the design of wiring of printed wiring boards, it is imperative to take measures to suppress noise during operation. In particular, electronic circuits that operate with high frequency are prone to generate noise and it is important to take sufficient measures. Conventionally one measure against noise during operation is in the manner of placement of bypass capacitors on the board.
In particular, in printed wiring boards with high frequency signal wires, absorption of high frequency power ripple that happens at the rising edge of a high frequency signal, in other words supply of high frequency current to an IC, is mainly performed by a capacitor. Therefore, in order to reduce electromagnetic noise and improve quality of wiring boards, it is desirable to place the capacitor as close as possible to the IC power pin. This kind of technique is described in xe2x80x9cPrinted Circuit Board Techniques for EMI Compliancexe2x80x9d, Mark I Montrose, IEEE Order No. PC5595 and xe2x80x9cHIGH-SPEED DIGITAL DESIGN A Handbook of Black Magicxe2x80x9d, Howard W. Johnson and Martin Graham, PTR Prentice-Hall.
Furthermore, conventionally in wiring board placement/wiring CAD, small components such as capacitors, coils (inductors), resistors, filters etc are assigned to large components such as ICs and connectors according to conventional component types or the state of net connection The components are placed by treating these sets of mutual components as one component, reducing the number of components to be considered on the wiring board overall. This kind of technique is described in xe2x80x9cA Method of Suppressing EMI from Printed Circuit Boards by Automatic Placement, Based on Limited Length of Critical Notesxe2x80x9d, Shinichi Tanimoto et. al, Shingaku Giho, EMCJ99-92, pp. 17-22, 1999.
However, when components are assigned based on component type, there is no differentiation between capacitors with high capacity for supplying low frequency current such as electrolytic capacitors and tantalum capacitors, and low capacity ceramic capacitors for supplying high frequency current in relation to main, large components (ICs), but rather all are simply treated as capacitors. For this reason, as with capacitors C6 to C8 (that have a lower capacity than C1 to C3) in FIG. 1, the capacitors are assigned in clusters. Therefore the component assigning is not carried out appropriately, requiring the user to make revisions. Moreover, capacitors that supply higher frequency current should be placed closer to the IC power pin. However, it has become common recently for ICs to have a plurality of power pins, and also many pin pair combinations of IC power pins and power pins of pluralities of capacitors, making selecting and wiring suitable pin pairs difficult.
A CAD apparatus for confirming whether the placement of a bypass capacitor is appropriate is disclosed in Japanese laid-open patent application H10-07560 (Computer Aided Design System).
This CAD apparatus makes confirmation of the placement of bypass capacitors possible by displaying on the board the effective range for eliminating noise for each bypass capacitor.
FIG. 2 shows a display example of a wiring board being designed displayed on the monitor of the CAD apparatus disclosed in Japanese laid-open patent application H10-07560.
At this point, a bypass capacitor 2110, an IC 2120, and an IC 2130 have been placed on a wiring board 2101 shown in FIG. 2.
The effective range for eliminating noise with the bypass capacitor 2110 is shown by an ellipse 2141 and an ellipse 2142. Here, the ellipse 2141 shows the range in which noise elimination by the bypass capacitor 2110 is highly effective, while ellipse 2124 shows the range in which noise elimination by the bypass capacitor 2110 is moderately effective.
Designers, referring to these effective ranges for eliminating noise, can confirm the placement of bypass capacitors by judging whether each pin of the components placed on a wiring board is in an effective range for eliminating noise by a bypass capacitor.
Here, the 4 pins on the left side of the IC 2120 are within the ellipse 2141, therefore noise elimination is estimated to be highly effective. The 4 pins on the right side of the IC 2120 are within the ellipse 2142, therefore noise elimination is estimated to be moderately effective. None of the pins of IC 2130 is within either the ellipse 2141 or the ellipse 2142, therefore noise elimination is estimated to be less than moderately effective.
However, in a display such as the above, as the number of bypass capacitors placed on a wiring board increases, the number of ellipses also increases. As a result, the ellipses overlap and the display becomes extremely difficult to see when the number of bypass condensers exceeds a certain level.
Furthermore, because each pin of a component has different operating characteristics, such as operation frequency, there are cases in which each pin conforms to a different bypass capacitor. In such cases, in a display such as the above-described, it is difficult to know which bypass capacitor is effective with which pin of which component, possibly resulting in errors in judgement.
In addition, capacitors have capacities, so even if the characteristics match, if the capacity is insufficient the noise elimination effect will be inadequate. However, in a display such as above, the capacity of bypass capacitors is not considered so it is difficult to detect if the capacity is insufficient.
Moreover, a judgement method using a display such as the above-described is inaccurate because the effectiveness of a bypass capacitor is judged by the linear distance on the surface between the bypass capacitor and a pin on the board. The reason for this inaccuracy is that the effectiveness of a bypass capacitor is not determined by the linear distance on a surface, but rather depends on the length of the path determined by the wiring pattern along which the harmonic content of the transient current flows.
It is a first objective of the present invention to provide a CAD apparatus for performing component placement to effectively reduce electromagnetic radiation noise.
It is a second objective of the present invention to provide a CAD apparatus for easily confirming whether bypass capacitor placement is suitable.
A CAD apparatus that achieves the above-described first objective includes a determining unit for determining a component order in an ascending order of impedance of passive components amongst components to be placed on the printed wiring board, and placement unit for placing the passive components in the determined component order.
According to this construction, the passive components are placed in ascending order of impedance. The lower the impedance of a component, the higher the component reduces high frequency noise, so passive components are placed in order from high frequency noise to low frequency noise. The earlier a component is placed, the greater freedom there is in placement, therefore, the higher the frequency of the noise, the more effectively a passive component can be placed to reduce the noise.
Here, the placement unit places each of the passive components in a vicinity of a power pin of a non-passive component which is already placed.
According to this structure, high frequency noise in power pins that is caused by current can be effectively reduced.
Here, the determining unit may determine the component order using an ascending order of equivalent series inductance of the passive components as the ascending order of impedance.
According to this construction, the equivalent series inductance is used instead of the impedance of passive components, so that different types of passive components can be treated in the same way.
Here, the determining unit may include a table unit for retaining a plurality of pin spacings of passive components and an inductance value corresponding to each pin spacing, a referring unit for referring to the equivalent series inductance corresponding to the pin spacing of each passive component retained in the table unit, and a sorting unit for sorting the inductance value referred for each passive component in ascending order, and making the ascending order of inductance value the component order.
According to this construction, the conversion unit utilizes the property that the lower the pin spacing is, the lower the equivalent series inductance is, and converts the pin spacing to the equivalent series inductance by referring to the table unit. Therefore, if the pin spacing of passive components is known, a component order can be easily determined for different types of passive components, even if the electrical characteristics of the components are not known.
Here, the determining unit may determine the component order using a descending order of effective frequency spectrum as the ascending order of impedance, the effective frequency spectrum being a frequency spectrum in which the impedance of a passive component is no greater than a threshold value.
According to this construction, the component order is determined in descending order of frequency at which passive components are effective against noise. Therefore, the passive components can be placed effectively in descending order of high frequency because the component order is the descending order of effectiveness.
Here, the determining unit may include a table unit for retaining a plurality of pin spacings of passive components, and an effective frequency spectrum corresponding to each pin spacing, a referring unit for referring each the effective frequency spectrum corresponding to the pin spacing of each passive component retained in the table unit, and an sorting unit for sorting the effective frequency spectrum referred for each component in descending order, and making the descending order of effective frequency spectrum the component order.
According to this construction, the conversion unit converts the pin spacing to the effective frequency spectrum by referring to the table unit. Therefore, if the pin spacing of passive components is known, a component order can be easily determined for different types of passive components, even if the electrical characteristics of the components are not known.
Here the determining unit may include a calculation unit for calculating the effective frequency spectrum for each passive component from at least one of a capacitance and an inductance of the passive component, and an sorting unit for sorting the effective frequency spectrum calculated for each component in descending order, and making the descending order of effective frequency spectrum the component order.
According to this construction, the effective frequency for each component is directly converted from the capacitance and the inductance, therefore the component order can be determined accurately in descending order of effective frequency spectrum.
Here, the calculation unit may calculate the effective frequency spectrum using at least the inductance of a passive component, when the passive component is one of a capacitor, a resistor, and a filter.
According to this construction, even when a passive component is one of a capacitor, a resistor, and a filter, it is possible to calculate the effective frequency spectrum from the inductance, therefore the component order can be determined accurately in descending order of effective frequency spectrum.
Here, the passive components may be capacitors, and the determining unit may determine the component order using an ascending order of the equivalent series inductance of the capacitors as the ascending order of impedance.
According to this construction, components are placed in a component priority order which uses the ascending order of equivalent series inductance of capacitors, in other words the descending order of frequency effective in noise reduction, as the component order. Therefore, design of a wiring board having good noise characteristics can be performed effectively.
Here, the determining may determine the component order using an ascending order of capacity of the capacitors as the ascending order of equivalent series inductance.
According to this construction, the property that the lower the capacity of a capacitor is, the lower the equivalent series inductance is, is utilized. Therefore, the component order can be determined easily.
Here, the determining unit may determine the component order considering an ascending order of terminal spacing of the capacitors to be the ascending order of equivalent series inductance.
According to this construction, the property that the smaller the terminal spacing of a capacitor is, the lower the equivalent series inductance is, is utilized. Therefore, the component order can be determined easily.
Here, the determining unit may include a table unit for retaining a plurality of pin spacings of capacitors, and an equivalent series inductance corresponding to each pin spacing, referring unit for referring to each the effective frequency spectrum corresponding to the pin spacing of each capacitor retained in the table unit, and an sorting unit for sorting the equivalent series inductance referred for each component in descending order, and making the sorted equivalent series inductances the component order.
According to this construction, the conversion unit converts the pin spacing to an effective frequency spectrum by referring to the table unit, therefore, if the pin spacing is known the effective frequency spectrum of capacitors can easily be converted.
Here, the passive components may be capacitors, and the determining unit may determine the component order using a descending order of effective frequency spectrum, the effective frequency spectrum being a frequency spectrum in which the impedance of a capacitor is no greater than a threshold value, instead of the ascending order of impedance.
According to this construction, the component order of capacitors is determined in descending order of frequency in which a capacitor is effective in noise reduction. Therefore, appropriate capacitors can be placed in order from high frequency noise to low frequency noise.
Here the CAD apparatus may further include a pin order determining unit for setting a pin order for each power pin of non-passive components in order of seriousness of noise that can occur in a current that flows through the power pin, and an assigning unit for assigning each passive component to a component which has a power pin, in descending pin order and descending component order, and the placement unit may each passive component in a vicinity of the power pin of the component to which the passive component is assigned, in the descending order of component order.
According to this construction, the assigning unit assigns pins in descending order of pin priority to passive components is descending priority order, therefore the placement unit places passive components in descending order with power pins in descending order of seriousness of noise that can occur, therefore component placement with good noise characteristics can be performed.
Here, the pin order determining unit may determine the pin order using a descending order of a signal frequency which is driven by a current which flows through the power pin, as the order of seriousness.
According to this construction, components having low impedance can be placed with power pins which are the source of high frequency noise.
Here, the pin order determining unit may determine the pin order using an order of shortness of one of a rising time and a falling time of a signal which is driven by a current which flows through the power pin, instead of the order of seriousness.
Here, the pin order determining unit may determine the pin order using an ascending order of shortness of the shorter of a rising time and a falling time of a signal which is driven by a current which flows through the power pin, as the order of seriousness.
According to this construction, the pin order of can be determined for power pins having noise which results from current which flows through the power pins in order of the seriousness of the noise.
Here, the pin order determining unit may determine the pin order using a descending order of an amount of consumed current of a signal which is driven by a current which flows through the power pin, as the order of seriousness.
According to this construction, the pin order can be determined using the property that the greater the amount of current a power pin consumes, the more serious noise caused by the pin is.
Here, the pin order determining unit may calculate a voltage waveform of a signal which is driven by the current which flows through the power pin, based on a voltage, a frequency, a rising time, a falling time, and a duty ratio of the signal and sets the pin order using a descending order of a maximum frequency of a voltage that exceeds a voltage threshold in the voltage waveform, as the order of seriousness.
According to this construction, the pin order determining unit can determine the pin order making the descending order of maximum frequency obtained from the waveform of the signal driven by the current which flows through the power pin, the order of seriousness.
Here, the pin order setting unit may determine the pin priority order of power pins connected to a net, for each net, and the assigning unit may assign components to be connected to a net to one net.
According to this construction, power nets are made independent, and passive components can be assigned to noise-causing power pins.
Furthermore, the CAD apparatus of the present invention is a CAD apparatus for a printed wiring board for placing a component belonging to a second type of components in a vicinity of a component belonging to a first type of components, including a first determining unit for determining a pin order in order of seriousness of noise that can occur in a current that flows through a power pin, for a power pin of each of the components belonging to the first type of components, a second determining unit for determining a component order in ascending order of impedance for each component belonging to the second type of components, and an assigning unit for assigning a second type component which is highest amongst the components in the component order that are not assigned, to a first type component having a power pin which is highest amongst the power pins in the pin priority that are not assigned. The first type of components includes active components, and the second type of components is passive components. The CAD apparatus of the present invention further includes a placement unit for placing each second type component in a vicinity of a first type component having the power pin to which the second type component is assigned, in the component order. The CAD apparatus of he present invention further includes a storage unit for storing sets of net information, each set of net information showing a net made up of a plurality of pins to be connected, a dividing unit for dividing, based one set of net information, a net whose power pins are to be connected into section nets, each section net corresponding to a component group made up of one first type component and at least one second type component assigned thereto, a selection unit for selecting, for each section net, a power pin of a component whose impedance is highest, from amongst the second type components connected to the section net, as a representative pin, and a wiring unit for wiring each section net independently, and for wiring so that a plurality of the representative pins are connected.
According to this construction, each power net is divided into section nets, and a net made up of representative pins connecting the plurality of section nets is wired independently. Therefore, the propagation of noise between section nets can be reduced.
A CAD apparatus that achieves the second object of the present invention is a CAD apparatus for displaying at least one component placement on a wiring board, and aiding an evaluation by a user of whether a placement of a position dependent component, whose effectiveness differs according to a placement position, is appropriate, the CAD apparatus including a design information storage unit for storing sets of position information which show the position of each component on the wiring board, a relationship information storage unit for storing sets of relationship information of the placement dependent component in relation with an effected component which is effected by the placement dependent component, and a display unit for displaying, according to one set of placement information, the position dependent component and the effected component which is in relation therewith in the relationship information in correspondence, in a user-recognizable state.
According to this construction, corresponding effected components and position dependent components can be displayed in correspondence. Therefore, the user can easily evaluate whether a placement of a position dependent component is appropriate.
Here, the display unit may display the related position dependent component and the effected component in correspondence by linking the components by a line.
Here, the display unit may link one of a pin of the position dependent component and a main body of the position dependent component with one of a pin of the effected component and a main body of the effected component, by a line.
According to this construction, corresponding effected components and position dependent components can be displayed linked by a line. Therefore, the user can easily evaluate whether a placement of a position dependent component is appropriate without the display becoming difficult to distinguish even when the number of effected components and position dependent components increases.
Here, the relationship information storage unit may further store an effectiveness showing a degree of an effect, and the display unit may further display the effectiveness stored by the relationship information storage unit in a user-recognizable state.
According to this construction, the effectiveness can be recognized by the user, therefore the user can take effectiveness into consideration when evaluating whether the placement of position dependent components is appropriate.
Here, the display unit may link the related position dependent component and the effected component in a display state which differs according to a degree of effectiveness.
According to this construction, different effectiveness"" can be displayed by different lines. Therefore, the user can easily evaluate whether a placement of a position dependent component is appropriate without the display becoming difficult to distinguish even when the number of effected components and position dependent components increases.
Here, the display unit may distinguish the degree of effectiveness by one of a line thickness, a line shape, a line color, a line shade, and a line pattern.
According to this construction, different effectiveness"" can be displayed distinguished by different line thickness"", line shape, line color, line shade, or line pattern.
Here, the CAD apparatus may further include a retrieval unit for retrieving, based on the sets of position information stored by the position information storage unit, the position dependent component and the effected component effected by the position dependent component; and the relationship information storage unit may store the retrieved position dependent component and the retrieved effected component in relation.
According to this construction, a position dependent component and the effected component that is effected thereby can be retrieved form the design information.
Here, the retrieval unit may retrieve a position dependent component and an effected component which are within a predetermined distance of each other.
According to this, a position dependent component and effected components within a predetermined distance of a placement dependent component can be stored in relation with the placement dependent component.
Here, the retrieval unit may retrieve, for each position dependent component, a predetermined number of effected components which are in a predetermined ascending order of closeness to the relevant effected component.
According to this, a predetermined number of effected components which are within a predetermined distance of a placement dependent component can be stored in relation with the placement dependent component in order, for each placement dependent component.
Here, the retrieval unit may further set an effectiveness which shows a degree of effectiveness according to the distance or the order.
According to this, effectiveness can be set according to distance or order, therefore precise relationship information taking effectiveness into consideration can be produced.
Here, the position dependent component may be a capacitor, the effected component may be a switching element on which a noise elimination effect is potentially had by a capacitor, and the retrieval unit may further retrieve within a range in which a capacity that is required by a switching element for noise reduction does not exceed a capacity of the capacitor.
According to this construction, a switching element within a range that does not exceed the capacity of a capacitor can be put in relation with the capacitor.
Here, the retrieval unit may further retrieve within a range in which a total value of capacities which a plurality of switching elements require for noise reduction does not exceed the capacity of the capacitor.
According to this construction, a plurality switching elements within a range that does not exceed the capacity of a capacitor can be put in relation with the capacitor.
Here, the retrieval unit may further retrieve within a range in which an amended value, which is a total value of capacities required by a plurality of switching elements for noise reduction multiplied by a ratio of the plurality of switching elements being switched simultaneously, does not exceed the capacity of the capacitor.
According to this construction, a plurality of switching elements made up of a greater number of switching elements can be put into relation with a capacitor more accurately.
Here, the retrieval unit may further retrieve only when a frequency characteristic of the position dependent component and a frequency characteristic of the effected component match.
According to this construction, more accurate relationship information can be produced by taking frequency characteristics into account.
Here, a distance used in the retrieval unit may be one of a straight line distance, a Manhattan distance, an actual wiring distance, and a path distance in which a loop area is a minimum.
According to this construction, one of straight line distance, Manhattan distance, actual wiring distance, and path distance in which loop area is a minimum may be selected according to the wiring state.
Here, the CAD apparatus may further include an extraction unit for extracting, from amongst the effected components or the pins of the relevant effected components stored in the position information storage unit, an effected component or a pin thereof that is not in relation with a position dependent component or a pin thereof in the sets of relationship information stored by the relationship information storage unit, and a display unit for displaying the effected component or the pin thereof extracted by the extraction unit, in a user-recognizable state.
According to this construction, an effected component or a pin of an effected component that is not related to any position dependent component can be displayed. Therefore, leaks in placement of position dependent components can be detected easily.
Here, the CAD apparatus may further include an extraction unit for extracting, from amongst the position dependent components or the pins of the relevant position dependent components stored in the position information storage unit, a position dependent component or a pin of an position dependent component that is not relation with an effected component or a pin of an effected component in the sets of relationship information stored by the relationship information storage unit, and a display unit for displaying the position dependent component or the pin of the position dependent component extracted by the extraction unit, in a user-recognizable state.
According to this construction, an effected component or a pin of an effected component that is not related to any position dependent component can be displayed. Therefore, leaks in placement of position dependent components can be detected easily.
Furthermore, a program which implements each of the above-described units on a computer is a program which realizes the above-described first and second objectives.